When Dipper and Mabel discover evidence that the reported town founder is a hoax, they set out to expose the historical cover-up and prove that Mabel's silliness is not a bad thing.

When Mabel befriends a cute merman trapped in the public pool, she's determined to help him out. Meanwhile, Dipper decides to volunteer as a lifeguard to see Wendy.

Li'l Gideon swindles the Mystery Shack away, forcing Grunkle Stan and the kids to move in with Soos. While Stan weighs how he can still care for his grandniece and nephew, they decide to take on Gideon.